Our condolences to the great Joan Rivers over the passing of her sister, lawyer Barbara Waxler. The comedian and host of “Fashion Police” on E! was due to be feted by 300 people at her 80th birthday celebration tonight at the Metropolitan Club with decorations by her close friend Preston Bailey — whose wedding to Theo Bleckmann Rivers recently officiated. But we’re now told the party has been postponed. A spokeswoman for Rivers confirmed her sister had passed away and her birthday celebrations are on hold.